1. Field of the Invention
This invention broadly relates to brackets used in orthodontic treatment. More specifically, the invention relates to an orthodontic bracket having a latch for releasably retaining an archwire in an archwire slot of the bracket.
2. Description of the Related Art
Orthodontic therapy is a specialized type of treatment within the field of dentistry, and involves movement of malpositioned teeth to orthodontically correct locations. Orthodontic treatment often improves the patient's occlusion and typically enhances the aesthetic appearance of the teeth.
Many types of orthodontic treatment programs involve the use of a set of tiny appliances and wires that are commonly known collectively as "braces". During such treatment programs, small appliances known as brackets are fixed to the patient's anterior, cuspid and bicuspid teeth, and an archwire is inserted into a slot of each bracket. The archwire forms a track to guide movement of the teeth to orthodontically correct locations. End sections of the archwires are typically captured in tiny appliances known as buccal tubes that are fixed to the patient's molar teeth.
Many types of orthodontic brackets have archwire slots that are open on one side for insertion of the archwire, and bounded on remaining sides by a central body of the bracket tiewings or other structure. Brackets that are intended to be bonded to the patient's front tooth surfaces often have archwire slots that are open either on a buccolabial side (i.e., a side facing the patient's cheeks or lips) or an occlusal side (i.e., a side facing the outer tips of the teeth) of the archwire slot. Some brackets, however, are intended to be fixed to the lingual side of the patient's teeth (i.e., the side of the teeth facing the patient's tongue) and in that instance typically have an archwire slot that is open on a lingual side or on an occlusal side.
Many orthodontists use ligatures to connect the archwire to the brackets and to urge the archwire into an orientation of seating engagement in the archwire slot. One type of commercially available orthodontic ligature is a small, elastomeric O-ring. Orthodontic O-rings are installed by stretching the O-ring around small wings (known as "tiewings") that are connected to the bracket body on the gingival side (i.e., the side facing the patient's gingiva or gums) and on the occlusal side of the archwire slot. Once installed, the O-ring ligature extends around the tie wings as well as over the labial side of the archwire in order to urge the archwire toward a lingual wall of the archwire slot.
Metal ligatures, such as ligatures made of stainless steel, are also used to retain archwires in archwire slots of brackets. Metal ligatures are typically made of a short section of initially straight wire. During installation, the wire ligature is hooked around the tiewings and extended over the labial side of the archwire, and end sections of the ligature are then twisted together to form a loop to retain the ligature in place.
Unfortunately, some orthodontists are not entirely satisfied with elastomeric and metal ligatures. Such ligatures are somewhat time-consuming to install, during initial installation and also during reinstallation whenever replacement of the archwire or ligatures is desired. As can be appreciated, a savings in the amount of time needed for ligation can help to reduce the total time that the practitioner must spend with the patient and consequently aid in reducing the overall costs of orthodontic treatment.
Other disadvantages are also associated with elastomeric and metal ligatures. For example, there have been reports that polyurethane elastomeric ligatures have exhibited deformation and force decay during the course of treatment. In some instances, elastomeric ligatures are stained by food and beverages consumed by the patient and become somewhat unsightly. Metal ligatures often have sharp ends that may retain plaque and food debris and also may increase the risk of cross-infection.
In an effort to overcome the problems associated with conventional ligatures, a variety of orthodontic brackets have been proposed having various types of latches for coupling the archwire to the bracket. Such brackets are also known as self-ligating brackets. The latch comprises a clip, spring member, cover, shutter, bail or other structure that is connected to the bracket body for retaining an archwire in the archwire slot.
Examples of self-ligating orthodontic brackets having generally U-shaped ligating latch clips are described in U.S. Pat. Nos. 3,772,787, 4,248,588 and 4,492,573. In general, the clip of such brackets is slidably mounted on the bracket body, and a dental explorer or other small-tipped dental tool is used to move the clip relative to the body when needed in order to open or close the archwire slot. A self-ligating bracket known as the "Speed" brand bracket also has a movable, generally U-shaped clip for ligating the archwire to the bracket.
Other types of self-ligating brackets have latches that resemble swinging shutters or closures that pivotally move between a slot-open and a slot-closed position. For example, U.S. Pat. No. 4,712,999 has a rotatable cover plate that is pivotally connected at one end to a tiewing of the bracket along one side of the slot, and is releasably engagable at the other end with a tiewing that is located along the opposite side of the archwire slot. Other orthodontic brackets with swinging latches are described in U.S. Pat. Nos. 4,103,423, 5,516,284 and 5,685,711.
U.S. Pat. Nos. 4,371,337 and 4,559,012 describe self-ligating orthodontic brackets having latches that rotate about the longitudinal axis of the archwire slot. The latches of these references have a somewhat cylindrical shape that is rotatably received in a mating, cylindrical channel, and an outwardly extending arm is provided to assist in rotatably moving the latch between a slot-open and a slot-closed position.
A self-ligating orthodontic bracket that is described in U.S. Pat. No. 5,711,666 has a ligating latch that comprises a flexible flat spring member. One end of the spring member is fixed to the bracket body on one side of the archwire slot, and the opposite end of the spring member has notches that releasably engage latch sears or catches when the spring member is moved to a slot-closed position. To open the slot, the notches are disengaged from the catches and the spring member is bent to an orientation sufficient to enable the archwire to be removed from the archwire slot.
Other types of self-ligating orthodontic brackets have latches that comprise essentially flat plates that are slidable between a slot-open and a slot-closed position. Examples of such construction are shown in U.S. Pat. Nos. 5,094,614, 5,322,435 and 5,613,850. In general, the sliding latches described in those references move in upright channels that are located buccolabially of the archwire slot.
Another type of self-ligating bracket that has been proposed in the past has a latch that is made of a section of wire material that functions as a bail. The orthodontic brackets described in U.S. Pat. Nos. 4,149,314, 4,725,229 and 5,269,681 have wire-like latches that swing between a slot-closed position and a slot-open position. The orthodontic bracket described in U.S. Pat. No. 4,260,375 has a wire latch that is slidable between a slot-open and a slot-closed position.
Many of the self-ligating brackets described above as well as commercially-available self-ligating brackets have structure that is intended to prevent unintentional movement of the latch. It is undesirable, for example, for the latch to accidentally move to a slot-open position during the course of treatment since the archwire may disengage the bracket once the slot has been opened. Once the archwire has separated from the bracket, control over movement of the associated tooth is temporarily lost. In such instances, the patient must return to the orthodontist's office so that the wire can be re-inserted in the archwire slot in order to resume treatment.
It is also often desired to restrain movement of the latch in an opposite direction in some manner so that the latch does not unintentionally detach from the body of the bracket when the latch is opened. For example, the orthodontist may move the latch of each bracket to a slot-open position during the course of treatment in order to replace the archwire with an archwire having different characteristics. If the latch becomes separated from the bracket body during such procedures, the practitioner must interrupt the procedure to retrieve the latch and replace it on the bracket body.
Self-ligating orthodontic bracket and archwire systems can be classified as either active or passive. In the passive system, the overall width of the archwire in a labial-lingual direction is less than the effective labial-lingual depth of the archwire slot when the latch is closed, resulting in a certain amount of space between the archwire and either the labial or lingual extent of the archwire slot. Some practitioners believe that the passive system presents less friction between the archwire and the bracket than would otherwise be possible, and that such reduced friction facilitates sliding movement of the bracket along the archwire to such an extent that overall treatment time is reduced.
By contrast, in active systems there is no space in a labial-lingual direction in the archwire slot between the archwire and the effective labial-lingual extent of the archwire slot. As a result, one side of the archwire of such systems is typically seated in the slot while the opposite side of the archwire is in contact with the latch. Oftentimes, the latch of such systems is resilient and functions as a spring member to urge the archwire to a fully seated position in the archwire slot. Many orthodontists prefer active self-ligating systems because the lack of free space in the archwire slot in a labial-lingual direction improves rotational control of the associated tooth in directions about its long axis.
In some instances, orthodontists change the size of the archwires as treatment progresses. For example, during early stages of treatment a small diameter round archwire may be used when some teeth are located a significant distance from their ultimate desired positions. In those instances, a relatively small diameter archwire can be bent sufficiently to engage each bracket without imposing undue force on the patient's teeth. As treatment progresses and the teeth move closer to their intended destination, the orthodontist may replace the archwire with a somewhat larger and stiffer archwire having a rectangular cross-sectional shape. Rectangular archwires provide somewhat better control over movement of the tooth as it is moved closer to its ultimate destination.
Certain self-ligating brackets have archwire slots with an effective size that functions either in the active or passive mode in accordance with the overall labial-lingual dimension of the selected archwire. As an example, there may be an effective space in the archwire slot in a labial-lingual direction when a small diameter archwire is used such that the system functions in a passive mode. The same system may function in an active mode when a larger archwire is selected.
However, the self-ligating brackets described above are not entirely satisfactory, in that the choice of an active system or a passive system is somewhat limited. For example, the practitioner may determine during the course of treatment that a passive system may be desirable even when relatively large archwires have been selected. However, the practitioner in such instances may not be able to use a relatively large archwire in a passive mode if the effective archwire slot is relatively small, unless each of the brackets is replaced with a bracket having a larger effective archwire slot.